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The alveolar-arterial oxygen gradient

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The Alveolar : arterial oxygen gradient is an index of the efficiency of gas transfer across the alveolar membrane. It is the difference in oxygen tension across the alveolar membrane.

  • A-a (gradient) = PAO2 (partial pressure alveolar O2) - PaO2 (partial pressure arterial O2)

The alveolar partial pressure of oxygen, PAO2 is given as:

  • PAO2 = (FiO2 - PCO2)/R where R is approximately 0.8 and FiO2 is 20 kPa on room air.

Otherwise the A-a gradient may be estimated by the formula:

  • PiO2 (inspired PO2) - ( PaO2 (arterial PO2) + PaCO2 (arterial PCO2) )

The partial pressure of inspired O2 is approximately its percentage in kPa.

The upper limit for this gradient varies with age between 1.5 kPa and 3.0 kPa. A gradient in excess of 3.0 kPa suggests impaired gas exchange.

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